1. Field of the Invention
The present invention concerns self-propelled four wheel drive agricultural machines of the type having hydraulic propulsion motors for each wheel.
2. Description of the Prior Art
In the prior art, harvesting machines of various types have employed hydraulic wheel motors, primarily because of the superior reliability they afford over mechanical drive systems in loose soil, sand or mud, and under other adverse conditions. In conventional hydraulic drive systems wherein individual wheel motors are used to power each of four support wheels, it is usually the case that the motors are in parallel connection across the supply and return lines from the hydraulic pump. As long as all wheels maintain about equal traction, the driving torque applied to the wheels is substantially the same. However, if one wheel loses traction and slips on the ground, the parallel hydraulic connections to the motors cause the slipping wheel to accelerate because the pressurized hydraulic fluid to the wheel motors follows the path of least resistance. Accordingly, the other motors receive less fluid and the torque to their respective wheels is reduced. When the vehicle regains full traction with all wheels, the wheel which was accelerated retains traction while turning more rapidly than the other wheels. This can cause the vehicle to yaw, or deviate from the desired steering direction. If a wheel completely loses traction it spins rapidly and none of the other motors will drive the vehicle. The spinning wheel must either be chocked or the vehicle may have to be towed to a location where all wheels have traction.
In the same type of vehicle having parallel hydraulic connections to the motors, it is usually the case that either the front pair of wheel motors or the rear pair of wheel motors are both hydraulically cut out of the circuit and mechanically uncoupled from the wheels for road travel. Thus, either the front pair of wheels or the rear pair of wheels are used for road travel as between harvesting fields. With the same volume of fluid used for energizing the motors for road travel as is used in the usual harvesting operation, the vehicle speed approximately doubles. However, if one of the two drive wheels now loses traction, the free wheel accelerates and the motor of the traction wheel is either partially or fully starved, and may stall. If one of the drive wheel motors is stalled, due to the other wheel losing traction in a two wheel driving mode, the vehicle cannot be moved at all. Even if the disconnected wheel motors are brought back into use, this may not assure that the vehicle can then be moved, and it will then have to be towed.
A further disadvantage of the type of two wheel road-travel driving system where the motors are in parallel connection is that many harvesting machines are extremely large and are purposely driven along the shoulder of the road so as to keep clear of the road traffic. This often places the right hand driving wheel on a shoulder surface of loose soil or gravel. As a result of the unequal traction between the pavement supported wheel, and the wheel on the shoulder, a condition is established which can adversely affect the driving capabilities of the two driving wheels in the same manner above described. On an ascending grade or a descending grade, an even more potentially dangerous condition can result. If the vehicle is traveling uphill and the shoulder-supported wheel loses traction, that wheel accelerates and can cause the wheel supported on the pavement to stall. Thus, in the absence of mechanical brakes, the vehicle cannot be stopped, and even when moving, the vehicle is difficult to steer. Even slight grades may be hazardous with a harvesting machine which weighs several tons, as many machines do.
Another drawback of four wheel drive harvesting machines employing hydraulic motors in parallel connection is that in field harvesting operations an unequal loss of traction for a wheel on each side of the vehicle can result in unequal driving force for the wheels which still have full traction. While this condition can be hydraulically compensated, such compensation is not precise. For example, (assuming a constant flow of fluid from the main pump, and fixed displacement wheel motors) parallel-connection hydraulic motors in a four wheel drive system may each be provided with a flow control system to limit the flow to a tractionless wheel and thus reduce the deceleration of the wheels having full traction. However, each flow control system must necessarily be set at a higher flow volume than the motor normally requires in full traction operation. As a result, a wheel losing traction will still draw more than its normal supply of fluid, and will thereby starve fluid from the other motors, and they will correspondingly decelerate. This causes unequal driving force to the driving wheels, and though the effect is diminished by a flow control system, it is not totally eliminated. Moreover, if the vehicle is provided with a reversible main pump for obtaining reverse directional movement of the vehicle, and for braking, a flow control valve must be used at each side of every motor.
The above discussed disadvantages of the conventional drive system, where the wheel motors are connected in parallel across the pump outlet line and the pump return line, are eliminated by the hydraulic circuit of the present invention, which:
(1) Prevents yawing of the vehicle if a wheel momentarily loses partial traction during four wheel drive in the harvesting field.
(2) Employs only two driving wheels on the paved side of the road for high speed travel on the road.
(3) Prevents acceleration of one wheel on one side of the harvester if that wheel loses traction, and causes the torque exerted by the other wheel motor on the same side of the vehicle to be increased to maintain driving traction.
(4) Provides the action in (3) when one driving wheel slips, either during four wheel drive field travel or during two wheel drive road travel.
(5) Achieves the foregoing improved results by a hydraulic system which is no more complex or costly than the presently employed systems.